With recent technological advances and increasing demand for mobile devices, there has been dramatically increased demand for secondary batteries as energy sources. Particularly, lithium secondary batteries are widely used at present for their high energy density and high voltage. A typical lithium secondary battery uses a lithium transition metal oxide as a cathode active material and lithium, a carbon-based material, a silicon-based material or a tin-based material as an anode active material.
Lithium has attracted the most attention as an anode for secondary batteries due to its low potential and high electrical capacity per unit weight. In actuality, lithium has been most widely used as an anode material for primary batteries for the past three decades.
However, lithium suffers from the disadvantages of unsafety and poor cycle characteristics due to dendrite growth during repeated charge/discharge cycles, thus being unsuitable for use in secondary batteries. Although extensive research has been conducted on the use of lithium in the fabrication of secondary batteries, many problems still remain unsolved.
Since Sony Energy Tech. (Japan) reported a hard carbon-based anode active material for a lithium secondary battery capable of replacing lithium in the early 1990's, many carbon-based anode active materials have been put to practical use. Batteries with a capacity above 350 mAh/g, which is close to the theoretical value, are realized at present.
Charge/discharge cycles in a carbon-based electrode proceeds through processes different from those of lithium in which electrodeposition and dissolution of lithium ions are repeated. That is, lithium ions are repetitively inserted into and removed from a carbon electrode. This procedure is commonly termed ‘intercalation/deintercalation’. The electrochemical reactions are affected by various factors, including the degree of crystallization, shape and crystal growth direction of carbon. Some carbon allotropes are known. Of these, graphite and hard carbon having an irregular arrangement structure are particularly suitable for practical use in lithium ion batteries.
However, an anode made of a carbon-based material has a theoretical maximum capacity of 372 mAh/g (844 mAh/cc) and thus has a limitation in capacity increase. This limitation makes it impossible for the anode to perform a sufficient role as an energy source of next-generation mobile devices that are rapidly being developed. Annealing of soft carbon at a temperature of 1,000° C. or less is advantageous in terms of high capacity (500-1,000 mAh/g), but the irreversible portion of the capacity is significantly large. Therefore, graphite having a large reversible capacity is generally used.
Silicon-based anode active materials are known to reversibly absorb and desorb a large amount of lithium through reactions between silicon or a cobalt, nickel or iron alloy of silicon and lithium to form compounds. In this connection, further investigations are currently in progress. A silicon-based anode active material has a theoretical maximum capacity of about 4,200 mAh/g, which is ten times greater than that of carbon-based materials. Therefore, a silicon-based anode active material is a promising material capable of replacing carbon-based materials due to its high capacity.
On the other hand, the theoretical electrical capacity of a tin-based anode active material is 990 mAg/g, which is 2.7 times greater than that of graphite electrodes. A tin-based anode active material is also currently in the spotlight as an anode active material capable of replacing graphite electrodes.
However, when silicon- and tin-based anode active materials react with lithium during charge and discharge, they undergo a significant increase in volume ranging from 200 to 300%. Due to this volume change, the anode active materials are separated from current collectors or the anode active material particles are broken into smaller pieces during continued charge and discharge, causing a loss in electrical contact. Further, the irreversible discharge capacity close to 50% of the initial capacity results in a marked reduction in capacity as the charge/discharge cycles proceed, leading to poor cycle life characteristics.
A typical anode composition for a lithium secondary battery comprises an anode active material, a binder and a conductive material. In this connection, Korean Patent Application No. 10-2000-0044901 A, which was filed on Aug. 2, 2000, enumerates various kinds of polymers, including polyacrylonitrile and polyvinylidene fluoride, for use in a lithium-sulfur battery undergoing rapid electrochemical reactions, and discloses selected combinations of the polymers as binders (see Examples Section). Korean Patent Application No. 10-1997-0063299 A, which was filed on Nov. 27, 1997, discloses a binder binding with an active material to produce an anode sheet of a lithium secondary battery (see page 7, lines 24-26).
As the charge/discharge cycles proceed, the volume of an anode active material is generally varied. This volume variation leads to a degradation in the performance (e.g., shortened cycle life) of a battery using the anode active material. The kind and the amount of a binder in an anode composition affect a change in the volume of an anode active material accompanied by charge/discharge cycles. Generally, a change in the volume of an active material tends to decrease in proportion to the amount of a binder.
The use of a binder in an excessively large amount for the purpose of reducing a change in the volume of an active material during charge and discharge can somewhat prevent the separation of the active material from a current collector, as mentioned above, but involves many problems, for example, an increase in the electrical resistance of an anode due to the electrical insulating properties of the binder and a decrease in the capacity of a battery due to the relatively small amount of the active material.
Thus, there is an urgent need to develop a binder that exhibits high adhesive strength and excellent mechanical characteristics to sufficiently withstand a large change in the volume of a silicon- or tin-based anode active material in a lithium secondary battery despite the use of a small amount of the binder.